This invention relates to the float process for making flat glass whereby a stream of molten glass is fed onto a pool of molten metal (usually tin). While the glass is floating on the molten metal in a softened condition, forces are applied to stretch the ribbon of glass to the desired thickness. Part of these forces are conventionally applied by means of rolls that extend through the sidewalls of the float forming chamber and engage marginal edge portions of the glass ribbon. These side engaging means serve to prevent the ribbon from unduly losing width as it is being longitudinally stretched, or in some cases they may be used to laterally stretch the ribbon. In the most common practice, the attenuating rolls engage only the top surface of the ribbon. Traction between the attenuating roll and the glass ribbon is limited by the buoyant force of the molten metal on the glass ribbon. Traction can be enhanced by providing knurled or toothed glass engaging surfaces on the periphery of the attenuating rolls, but in some cases the traction attainable is not as great as would be desired, particularly when attempting to attenuate the ribbon after it has cooled substantially (e.g. below about 1500.degree. F., 820.degree. C.).
In the past it was also known to grip marginal edge portions of a glass ribbon in a float bath between upper and lower driven rolls, the lower roll being submerged in the molten metal. Such an arrangement would be capable of considerable traction, but that type of attenuating device has been found to be difficult to manipulate and also presents a difficulty in maintaining a fixed gap between the top and bottom rolls because of the potential for eccentricity of the roll shafts in the hot environment of the float forming chamber. Cooling the roll that is immersed in the molten metal has the drawback of a strong cooling effect on the adjacent molten metal. An attempt to overcome this problem is shown in U.S. Pat No. 3,373,008 (Lawrenson et al.) in which the lower roll is isolated from the molten metal by a protective slipper. Such an arrangement, however, is even more cumbersome and disadvantageously entails contact between the glass ribbon and the slipper member, whereby the marginal edge portion of the glass is significantly displaced. Deformation of the glass ribbon would be particularly undesirable when attenuating the ribbon after its viscosity has increased substantially in the cooler regions of the float forming chamber.